VEGF plays an undisputedly critical role in vessel growth. Though several biological actions of VEGF have been demonstrated, the precise manner in which VEGF governs vasculogenesis and /angiogenesis is not known. Furthermore, VEGF exists as at least three distinct isoforms. Using a genetic approach we have demonstrated that the isoforms are not functionally equivalent. The individual isoforms display differential binding to neuropilin and to heparan sulfate and this difference may form the basis for the distinct vessel phenotypes that arise from the expression of single VEGF isoforms. Finally, VEGF is expressed at high levels in adults organs where there is no active angiogenesis, suggesting that VEGF may play an as yet undefined role in the adult vasculature. With this background as rationale, we proposed the following aims. (1) To understand the role VEGF isoforms in tumor vascularization. The role of stromal VEGF will be determined by analyzing tumor growth and vascularization in animals in which wildtype (wt) tumor cells are implanted into adult isoform-specific mice. The role of tumor-derived VEGF will be assessed by implanting transformed mouse embryo fibroblasts cells homozygous for a single VEGF isoform into wt mice. The effect of a single isoform from both the host and stromal compartment will be determined by investigating tumor growth in mice derived by crossing isoform-specific mice with mice that spontaneoulsy develop pancreatic beta cells adenomas in situ (Rip Tag). (2) To investigate the mechanisms that underlie the distinct vessel phenotypes that result from expression of a single VEGF isoform. The role of neuropilin will be investigated in an in vitro metatarsal angiogenesis assay by blocking NP binding and by investigating vessel growth in mice that are both NP deficient and that express single VEGF isoforms. (3) To determine if VEGF plays a role in the maintenance of the adult vasculature. Mice will be engineered to inducibly express a dominant negative (DN) VEGFR2 in the endothelium. Expression of the DN at various times postnatally and in the adult will allow us to assess the permeability.